Various methods and devices are known for producing a predetermined concentration of a selected gas within a carrier gas flow. For example, a flow dilution technique may be used where the selected gas is stable and may be stored in a known concentration within a pressure vessel. One such gas is carbon monoxide (CO).
To perform the flow dilution technique, a first mass flow controller is used to establish a predetermined flow rate for the selected gas from a pressure vessel and a second mass flow controller similarly establishes a flow rate for a carrier gas. The carrier gas may be supplied from the atmosphere by means of a pump or from a pressure vessel depending upon the type of gas required. The flow of the selected gas is allowed to combine with the flow of the carrier gas, thereby diluting the selected gas to a concentration that is determined by several factors, including the stored concentration of the selected gas and the flow rates determined by the mass flow controllers. The accuracy of this technique depends at least in part upon the accuracy to which the known concentration of the selected gas in its pressure vessel may be determined, the purity of the carrier gas and the accuracy of the mass flow controllers.
Where the selected gas cannot be successfully compressed and stored in an accurately known concentration within a pressure vessel, as with a gas that may be reactive or unstable, a suitable gas generator may be employed to provide the selected gas within a carrier gas flow.
One such gas is ozone (O.sub.3) which may be produced by exposing oxygen (O.sub.2) to ultraviolet light having a wavelength of approximately 186 nm. Typically, an ozone generator using this technique may include a chamber through which a carrier gas including oxygen is passed. An ultraviolet lamp is disposed within the chamber and emits ultraviolet light in proportion to the power that is applied to the lamp. The ultraviolet light photochemically reacts with a portion of the oxygen and thereby forms ozone within the carrier gas flow. The ozone concentration produced by such an ozone generator, however, is in most instances not constant, i.e., drifts, and thus such a generator is generally not a suitable source for precise concentrations of ozone. This drift may result from, for example, fluctuations in the lamp intensity as the lamp ages, variations in the power that is applied to the lamp, inadequate circulation of the carrier gas around the lamp within the chamber, variations in the concentration of oxygen within the carrier gas flow, and temperature and pressure variations within the generator.
An ozone generating apparatus that attempts to minimize drift is disclosed in U.S. Pat. No. 3,752,748 to McMillan, Jr. The apparatus includes an ultraviolet lamp within a chamber and a detector which senses the intensity of the ultraviolet light emitted by the lamp. The power applied to the lamp is then adjusted to compensate for detected variations in this intensity. A telescoping tube mechanism is disposed around the lamp and by adjusting the length of the tube, the quantity of ultraviolet light within the chamber may be controlled to thereby vary the ozone concentration produced in the carrier gas. This apparatus remains subject to undesirable drift because, for example, the sensitivity of the detector may vary as it ages and as it is subjected to temperature variations. Also, the telescoping tube mechanism is relatively complex to manufacture and may introduce inaccuracies into the ozone concentration because of mechanical wear associated with the various components.
A second gas that may be considered exemplary of gases that cannot be readily stored is nitrogen dioxide (NO.sub.2). A first technique that may be used to introduce nitrogen dioxide gas into a carrier gas flow is known as permeation, wherein liquified nitrogen dioxide is sealed within a permeable tube which may be of teflon. For the particular tube, a permeation constant may be determined which relates the temperature of the tube to the rate at which the nitrogen dioxide permeates the the tubing walls. By adjusting the temperature of the tube, the permeation rate varies and an adjustable concentration of nitrogen dioxide may be introduced into a carrier gas which is allowed to flow around the tube. Accurate temperature control of the tube is difficult, however, and the tube itself may not display the permeation characteristics upon which the accuracy of the device may depend.
A second technique that may be used to introduce nitrogen dioxide into a carrier gas flow is gas phase titration. As is well known to those skilled in the art, this technique is based on the following empirical chemical equation: EQU O.sub.3 +NO.fwdarw.NO.sub.2 +O.sub.2
With this technique, excess nitric oxide (NO) is allowed to react with a predetermined concentration of ozone provided by an ozone generator. Although the above equation predicts that one molecule of nitrogen dioxide will be produced for every molecule of ozone consumed until no ozone remains, various factors, including reaction time, influence the completeness of this reaction. Also, the ozone generator will be subject to drift as described above which will result in a corresponding drift in the NO.sub.2 concentration.